<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>

<body>
    <div class="box"></div>
</body>

</html>

<style>
    .box {
        position: relative;
        background: red;
        height: 100px;
        width: 100px;
        -moz-border-radius: 50px;
        -webkit-border-radius: 50px;
        border-radius: 50px;
    }

    .box:before,
    .box:after {
        content: '';
        position: absolute;
        border-bottom: 70px solid red;
        border-left: 40px solid transparent;
        border-right: 40px solid transparent;
        top: 70px;
        left: -10px;
        transform: rotate(-140deg);
    }

    .box:after {
        left: auto;
        right: -10px;
        transform: rotate(140deg);
    }
</style>